Engineering geology
Igneous rock

Igneous Processes

Igneous rocks

Igneous rocks form in two basic settings. Molten rock may crystallize within Earth’s crust over a range of depths, or it may solidify at Earth’s surface.

Igneous rock
Intrusive versus extrusive igneous rocks

When magma crystallizes at depth, it forms intrusive igneous rocks, also known as plutonic rocks—after Pluto, the god of the underworld in classical mythology.

These rocks are observed at the surface in locations where uplifting and erosion have stripped away the overlying rocks. Exposures of intrusive igneous rocks occur in many places, including the White Mountains, New Hampshire; Stone Mountain, Georgia; Mount Rushmore in the Black Hills of South Dakota; and Yosemite National Park, California.

Igneous rock
Mount Rushmore National Memorial This memorial, located in the Black Hills of South Dakota, is carved from intrusive igneous rocks. (Photo by Barbara A. Harvey/Shutterstock)

Igneous rocks that form when molten rock solidifies at the surface are classified as extrusive igneous rocks. They are also called volcanic rocks after Vulcan, the Roman fire god. Extrusive igneous rocks form when lava solidifies or when volcanic debris falls to Earth’s surface. Extrusive igneous rocks are abundant in western portions of the Americas, where they make up the volcanic peaks of the Cascade Range contrast, and the Andes Mountains. In addition, many oceanic islands, including the Hawaiian chain and Alaska’s Aleutian Islands, are composed almost entirely of extrusive igneous rocks.

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